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One Size Didn't Fit All for Early Dinosaur, Study Says

James Owen
for National Geographic News
December 15, 2005
 
One size didn't fit all for the early dinosaur Plateosaurus, a new study suggests.

Fossils show the giant plant-eaters experienced sudden growth spurts, with some adults dwarfing others.

The study team estimates that the biggest individuals measured 33 feet (10 meters) in length and weighed almost 4 tons. Other dinos of the species were twice as small, managing an adult body length of only 15 feet (4.8 meters).

Researchers say the animal, which lived some 200 million years ago, had growth patterns like those seen in living reptiles but unlike those of other, later dinosaurs.

Scientists at the University of Bonn in Germany analyzed growth rings found in the fossilized leg and pelvic bones of Plateosaurus, a long-necked, two-legged dino once common across Europe.

The study, which appears tomorrow in the journal Science, suggests that adults grew at different rates and that their growth occurred either in rapid spurts or slow stretches depending on environmental conditions.

The scientists add that this growth model in such a large dino species means that Plateosaurus possibly marks an initial step in the evolution of genuinely warm-blooded dinosaurs.

Size Range

Mammals grow "in accordance with a genetically programmed blueprint," said Martin Sander, a University of Bonn palaeontologist and lead study author.

Humans, for example, can reach different sizes as adults, but individuals' growth rates will be fairly predictable over time.

Dinosaurs were thought to have had steady growth patterns similar to mammals, but "our findings have thrown this conception into disarray, at least for one dinosaur," Sander said.

Sander and his colleagues say Plateosaurus probably had a similar metabolism to living reptiles. Like today's lizards, crocodiles, and turtles, the dinosaur's growth "was affected by environmental factors such as climate and food availability," the researchers report.

Some individuals hit upon excellent conditions for piling on the pounds, while others fell on lean times.

Dinosaur researcher Paul Barrett, of the Natural History Museum in London, agrees that the study appears to provide "nice evidence of some kind of temporary growth spurt rather than continued, similar rate of growth throughout the animal's lifetime."

This "might correlate with greater instances of food or maybe a warmer summer," Barrett said. "That would make sense in interpreting [Plateosaurus] in a crocodile- or lizardlike model."

Growth spurts and big size differences within a species are characteristic of ectothermic animals—those with a metabolism dependent on external conditions. Lizards, for instance, are active when it's hot but become sluggish in cold weather.

Meanwhile mammals and birds, which have growth rates largely independent of outside factors, are endothermic—they heat and cool their bodies internally.

Which of these two camps dinosaurs belonged to has long been a bone of contention among experts.

Sander says Plateosaurus was probably somewhere between the two, with the animal perhaps representing a first stage in the evolution of endothermic dinosaurs.

Warm-Blooded

Barrett of the Natural History Museum says early large dinosaurs such as Plateosaurus were not endothermic, but likely were warm-blooded.

"They're probably warm-blooded because they're big and they produce a lot of heat through digestion and through muscular action," he said.

"They have a relatively small surface area to their volume ratio, and any excess heat they generated just by accident would mean they have a higher body temperature than the outside environment."

Based on their body structure, some later, smaller dinosaurs also appear to have been endothermic.

"It's a size argument," Barrett said. "Because they are small and active, it looks like they should be warm-blooded to fuel their active lifestyle."

"Some of these little dinosaurs are now known to have had an insulating covering—feathers or fuzz of some kind," he added. "This suggests they were generating heat internally and were trying to retain it."

These small theropod dinosaurs are on the evolutionary line that leads eventually to birds, Barrett says.

"Certainly, by the time you get to birds, [the animals] are generating heat internally," he added.

"Birds are only modified dinosaurs, and at some point in their ancestry you need to switch [endothermy] on. Deciding when that happened is quite difficult, as we can't just go and measure the temperature of an extinct dinosaur."

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